The present invention relates to a radiating coaxial high frequency cable which is comprised of an inner conductor, and an outer conductor holding, for example, the inner conductor in concentric relation by means of dielectric spacer means.
Cable of the variety to which the invention pertains are used, for example, in a manner which combines the transmission function for h.f. energy with the radiating (or receiving) function of an antenna. For example, the cable is laid along a particular path such as a railroad track and a mobile receiver picks up signals radiated from this antenna cable. This mode of transmission is used, for example, inside of a tunnel, to avoid interruption of communication with a train passing through. The cable is laid on the ties, next to the track or along the tunnel's wall. The transmission may well occur in the reverse direction, the cable functioning as a receiving antenna, the transmitter being mobile.
In order to provide for the transmission, of radiation out of the cable, the outer conductor is usually of open construction. For example, German printed patent application No. 10 44 199 discloses a coaxial cable whose outer conductor has an axial, longitudinal slot. This cable is of rather simple construction and can be made quite easily. The cable serves as wide band transmitter antenna and is satisfactory if the signal to be received does not have to be very uniform. It was found that the external electrical field set up by the slot varies significantly in strength on account of superpositioning of several cable modes. The amplitude of the signal, when received on and along the outside of the cable, varies strongly accordingly.
German printed application No. 16 90 138 discloses a radiating h.f. cable whose outer conductor is of tubular configuration, and the tube has spaced-apart slots for emitting an electromagnetic field. The slots vary in direction and form and establish a non-continuous zig-zag line. This arrangement of slots is to suppress the axial component of the field and enhances the radial component as far as radiating transmission is concerned. Length, width and angle of inclination of these slots are parameters determining the strength of the radiating field, and the spacing of the slots determines the transmission band. Generally speaking, different transmission characteristics require different slot arrangements, patterns, dimensions, etc. This is particularly true on account of the fact that such a cable has a narrow bandwidth as far as its antenna function is concerned. Thus, the purpose of a particular cable is limited and different cables, for different purposes, are to be made differently so that one needs different tooling. Generally then, the making of such a radiating cable is not very economical.
U.S. Pat. No. 3,795,915 describes a radiating cable whose bandwidth has been enlarged by using many slots which differ in length and orientation, there being a periodically repeated pattern. Indeed, this cable can be used for operation over a wider frequency band, but due to the complex slot pattern, it is quite expensive to make.